In urological function diagnostics, it is often necessary to measure the flow of a patient's urine (uronecessar flowmetry), in order to determine by the analysis of a uroflowmetry curve, the extent of a micturition disorder. For producing such a curve there is needed a measuring instrument capable of supplying a sufficiently accurate, continuous and reproducible recording of the measured flow values.
Urine flow and micturition volume can be measured, for example, by gravimetry, rotation dynamics, or electrical capacitance or inductance. In gravimetry the weight of the micturition volume is measured and to this end, the urine passed during micturition is collected in a graduated cylinder and weight gains thereof are continuously measured and recorded.
In the rotation dynamics method, the urine stream strikes a disk rotated by a driving motor about vertical axis, thereby causing the disk to be braked. An electronic control unit ensures that the speed of rotation of the disk remains constant and the electric power consumed by the driving motor provides an indication of the quantity of urine striking the disk and hence an indication of the urine flow. The urine is accelerated on the disk and is finally flung against the wall of a housing which serves as a collecting vessel.
Inductive throughflow measurement uses the effect of a magnetic field upon moving matter. Urine, an electric conductor, is moved in the magnetic field, so that an electric voltage is induced in the urine. This induced voltage is, for liquids, proportional to the mean flow rate thereof. In order to measure the throughflow, two electrodes are mounted at right angles to the magnetic field in an electrically non-conductive section of the tube, the induced voltage is measured at these electrodes.
In the case of capacitive uroflowmeters, for example according to the teaching of DE-B 25 00 094, U.S. Pat. No. 4,051,431) a fluid
level measurement is converted into a change in capacitance. To this end, a generally cylindrical electrical capacitor is mounted in a collecting vessel for the urine to be measured. The capacitance of this arrangement varies with the level to which the vessel is filled and is therefore a measure of the filling level in relation to time and thus of the volume function, from which the flow of urine can be determined by means of electrical differentiation.
There is described in DE-A-23 30 033, an arrangement in which the electrolytic conductivity of the urine in a collecting vessel is used to measure the fluid level. An uninsulated electrical resistor is partially shunted out or short-circuited relative to an external electrode by the urine. The resistance of said resistor therefore, varies with the height of the fluid level in the vessel and is accordingly a measure of the level to which the vessel is filled.
A disadvantage of most uroflowmeters operating on electrical principles is that each time such a device has been used it must be emptied and cleaned, a laborious process which may give rise to sources of error. Where the resistance of an electrical resistor is measured, as described above, the turns thereof may, for example, be short-circuited by urinary crystals, so that measuring errors can arise, or conductive coatings may form between the test electrodes. Uroflowmeters operating on the principle of rotation dynamics, have apart from cleaning problems, the further disadvantage of being relatively expensive, since a driving motor for the disk and a bearing system therefore, must be provided. In inductive throughflow meters, the measuring signal is relatively small and the electromagnet which is necessarily powerful is relatively expensive.